Build on Linux (Ubuntu 16.04) with AppVeyor. Disable PowerShell on
Linux. Ubuntu 16.04's dependencies are different from the existing
Ubuntu 14.04 dependencies, so a separate script for installing those
dependencies has been added.

AppVeyor commands use cmd/batch by default. Specify cmd for lines meant
for cmd to differentiate them from lines meant for ps/PowerShell or
sh/shell/Bash.

Move the AppVeyor install scripts to a separate script file. This
shortens the appveyor.yml file and makes it easier to introduce new
install scripts for other build environments.

There is one minor change to the install script, and that was to change
directory back to the obs-studio git root directory.

AppVeyor recently removed QT 5.7.1 for the Visual Studio 2015 image.

This is not necessary with the updated pre-built windows dependencies.
This reverts commit 516e9daa.

Closes jp9000/obs-studio#1079

If all retries fail, stop the build.

Currently this build is still missing the following plugins:
enc-amf
obs-browser
win-ivcam